Since the Wright Brothers first proved that heavier than air aircraft are feasible, man has worked on improving the performance of the same.
One of the problems encountered has been that high speed performance has been at the sacrifice of low speed performance and vice versa. Aircraft that perform well at high speed have a proportionally higher landing and stall speed while aircraft that perform well at low speed and have slow stall characteristics are incapable of high speed.
One of the attempted solutions to the above-indicated problem has been the development of swing wing aircraft which extend their wings during relatively low speed and partially retracted the same during high speed.
The expense of manufacturing the highly sophisticated structural means to support the swing wing as well as the complicated controls required to operate the same effectively prices this concept out of all except the supersonic jet aircraft market.
More specifically, all finite lifting surfaces such as aircraft wings, canards, horizontal tails and the like experience a flow phenomena at their tips known as the roll-up of tip vorticies. These vorticies occur throughout the angle of attack range from the lowest angle up to the highest where flow separation and surface stall occurs, especially in the region of the tips. The strength of these vorticies increases as the angle of attack and surface loading increases.
Tip vorticies are formed when high pressure air acting on the lower surface of the lifting structure tends to flow spanwise toward the tip and into the ambient low pressure area outside the tip. This spanwise flow tends to spill up and around the tip and back into the low pressure area behind the lifting surface, thus forming a tip vortex. Energy is lost at the formation and shedding of these tip vorticies. The tip vorticies also produce an upwash at the tips which causes high angles of attack at such tips resulting in early tip stall and associated energy losses.
Roll up of air flow at the tips represents a considerable loss of energy into the air stream which manifests itself as a reduction in lift accompanied by large increases in drag, especially when flow separation and stall occurs and in fact at stall angles of attack or angles near stall, these effects can be quite severe. For example, on aircraft which normally take off and land at high angles of attack, tip stalls can occur resulting in catastrophic losses in lift, large increases in power required, and in severe stability and control problems.
Over the years, various devices and design techniques have been used to help overcome some of these problems. One traditional method of delaying or moderating tip stall is the use of negative twist or wash out on wings and other lifting surfaces. Surface wash out reduces the angle of attack at which the tip airfoil sections operate thus allowing the tip areas to stall later than does the inboard area.
Another technique or method used to improve the tip flow characteristics is the use of end plates mounted vertically on the tips of the lifting surfaces to block or impede the spanwise air flow thus reducing the strength of the tip vorticies and thereby improving the tip stall characteristics. This flow blockage effectively increases the aspect ratio of the lifting surface which in itself improves the lift and drag characteristics. Although currently several aircrafts are achieving some success with end plates mounted vertically on their wing tips, this has certainly not proved to be a complete answer to the problem.